ushembo,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css"> div{
width: 100px;
height: 100px;
}
.Red{
background-color: Red;
}
.Green{
background-color: Green;
}
.Yellow{
background-color: Yellow;
}
</style>
</head>
<body>
<div class="form-group bx-soa-customer-field" data-property-id-row="15"></div>
<div class="form-group bx-soa-customer-field" data-property-id-row="12"></div>
<div class="form-group bx-soa-customer-field" data-property-id-row="18"></div>
<script>
document.querySelector('[data-property-id-row="18"]').className ='Red';
document.querySelector('[data-property-id-row="12"]').classList.add('Green');
document.querySelector('[data-property-id-row="15"]').setAttribute('class','Yellow');
</script>
</body>
</html>